The following relates to the optical filter arts, optical characterization arts, and related arts, and to applications using same.
Optical filters are used in a wide range of optical applications, such as astronomy, still and video cameras and other imaging devices, televisions, computer monitors, cellular telephone screens, and other display devices, optical sensors such as motion sensors, gesture sensors, and the like, and so forth. Depending on the application, such filters can range in size up to a square meter or larger in area. Within a design-basis optical spectrum an optical filter generally has a passband for which the filter transmits light, and a stopband (or blocking band) for which the filter does not transmit light. In a bandpass filter, the passband is usually relatively narrow and most of the design-basis optical spectrum lies outside of the passband and is blocked. Passband filters with two or more separate passbands are also known. In a low-pass or high-pass filter, the passband includes all wavelengths of the design-basis spectrum below (or above) a cutoff wavelength or frequency. Filters can be further optically characterized by parameters such as the transmission in the passband and the stopband, passband full-width-at-half-maximum (FWHM), the slope of the transition at the edge(s) between the passband and the stopband, and so forth.
Optical filters can employ various designs. A common type of optical filter is an interference filter, in which a stack of optical layers are arranged with thicknesses and refractive indices that are precisely designed so that reflected and transmitted light rays within the stack constructively combine within the passband and destructively combine in the stopband. An interference filter can provide steep passband edges, high passband transmission (approaching 99% or higher) and very low transmission in the stopband (near 0% transmission). The stack of layers is typically formed on an optically transparent substrate (e.g. glass) by a technique such as sputter deposition, thermal vacuum evaporation, or the like which is performed in a vacuum chamber or other controlled atmosphere.
The manufactured filter is typically visually inspected for defects, and its filter characteristics are measured, usually by spectral measurement over the design-basis wavelength range, to confirm the filter specification is met (e.g. passband center wavelength and FWHM, cutoff slope, et cetera). Filter characterization is usually performed for each manufactured filter because for filters with demanding specifications filter characteristics can change significantly (enough to be out-of-specification) in response to even small errors in layer thicknesses or refractive indices. Since thickness non-uniformity over the area of the filter can occur, the filter spectral characterization may be repeated at several different areas. Those filters which pass inspection and meet the filter specification are delivered to the customer, or alternatively may first be mounted in a filter frame or optical sub-system before delivery.
Some improvements are disclosed herein.